Electronic noise generator



Dec. 6, 1949 s s STEVENS 2,4903187 ELECTRONIC NOISE GENERATOR Filed Oct. 29, 1945 l Pan/fe wfz/f/e daan/A1775 I Patented Dec. 6, 1949 vii:A

TI have'also'foudithat'fa -gasilied'ldiodevmaybe utilized to generate a noise spectrum whieii'isrticularly suitable to constituting -the .random or continuous' fs'iitni eoin'ponent f 'airplane noise.

I In the gas-'filled diode, eurrent'iscarriedlprincithat the max'iin'in 't'p't Will? befpo'ducel when such a tube is operating near theeritical-yioniza tion point. In applying suh, potentials -to purposes of this invention, it.ishecessary-irst to isolate and amplify this noise; andthen tofpass it through such filtering networks as will give the proper distribution kof "eneifgyiin the Various "vreems-@ftheSiouiitirum i u o .i

L'Irl-Fig." zbf-thevfdiawiga "I-have-ilistrteda suitable wiring diagram of a circuit in which a gas-lled diode connected triode is utilized to generate a random spectrum. The output of this generator is led to a preamplier and then to the same power amplifier into which the output of the line spectrum generator is passed (Fig. 1).

In Fig. 3 of the drawings, I have illustrated one suitable wiring diagram of a circuit in which a relaxation oscillator is utilized to generate a line spectrum. The output of the line spectrum generator, as illustrated in the block diagram of Fig 1, is passed through filters to a preamplifier and then to a power amplifier where it is mixed with the output of a random spectrum generator. The iilters act in a manner well understood in the art toselect relatively more of the harmonics lying between 100 cycles and 250 cycles and to attenuate markedly frequencies lying above 250 cycles. The

amplification and mixing is accomplished by familiar electronic methods, the detailed manner of operation of which is immaterial to the invention. The combined output of the two-generators i passed from the power amplifier to a frequency control then to a dividing network and finally to high frequency and low frequency speakers as has been illustrated in the block diagram of Fig. l.

The frequency control network is capable of emphasizing a particular group of components which have been found to closely simulate similar components occurring in average airplane noise. This particular band of frequencies from the line spectrum generator has been found to occur between 100 cycles and 250 cycles. The selection of desired frequencies from the line spectrum generator was accomplished as shown above by a band-pass filter. It remains at this point to attenuate in smaller measure (than the band-pass filter) the high frequency components of the output of the random spectrum generator. It was found possible to accomplish this with conventional tone control of the power amplifier although it might equally well have been done with other known filtering networks. This tone 'control consists of a condenser in series with a resistor. By utilizing the voltage across the condenser, the output decreases proportionally to the frequency because of the decreasing impedance of the condenser at high frequencies. This combination of resistance and capacitance for this purpose is common practice in the art.

The numerical values of the elements which have been found to produce a desirable noise are given below although these values may be changed to produce noise over a greater frequency band without departing from the spirit and scope of the invention.

Any good power amplifier with a wide range of frequency response and a reasonably small distortion willadequately meetlthe needs of this'noise generator. It should have good dynamic power handling capacity. The power needed will depend principally upon the sound level to be achieved, and upon the characteristics of the room in which the noise is to be produced. For example, a battery of six 50-watt power amplifiers is sufficient to produce an overall noise level of better than 120 db. in a room of 10,000 cu. ft. volume, with hard plaster walls. For most purposes, commercially available 50-watt power amplifiers are quite suitable.

If more than 50-watt power is required, these units can be used in parallel with suitable adjustment of the output circuit to match the impedance of the speakers used. Loudspeaking equipment of the type particularly utilized in systems for theatre application have been found to f vrgive best results.

Current engineering practice indicates the desirability of using a dividing network and both a low frequency and a high frequency horn. Satisfactory results have been obtained from a system which includes a Jensen vPLJ--18B in a bass reflex cabinet to handle frequencies up to 400 cycles. This apparatus was set up in a brick-walled room of 2500 cu. ft. volume.

The noise generating equipment described is effective in synthesizing an average noise generally corresponding to the noisefield in which the pilot of a plane is required to operate. By simulating a noise iield by means of electronic generation, more realistic training conditions are available to student pilots and others. This is especially useful in determining the ability of the pilot to hear and understand directions received in the earphones or other types of communication equipment. Electronically generated noise can also be made use of in various other training situations, covering a variety of types of equipment.

Having thus described my invention, what I claim is:

l. An electronic device for producing audio frequencies simulating the audio frequencies of the noise produced by aircraft that are adapted to be audibly produced over low frequency and high frequency sound producing means comprising a circuit including a gas-filled triode tube for producing voltages over a random frequency spectrum corresponding to the frequencies of air turbulence about an airplane in fiight, a circuit including a relaxation oscillator for producing voltages of a fundamental frequency and harmonics thereof, a band pass filter in circuit with the relaXation oscillator for selecting the harmonics corresponding in frequencies to harmonics present in propeller noises, means for amplifying the output voltages of said gas-filled tube, means for amplifying the voltages passed by said band pass lter, means for combining the outputs of said amplifying means, a power amplifier, means for :applyingthe combined-outputs of said amplifying means to said power amplifier, a network having -an adjustable frequency transmission characteristic, means for applying the output of-said power amplifier to said network, and means for -applyf ing the output of saidnetwork to'an audio frequency dividing network for separating the out: put frequencies into a low frequency band output and a high frequency band output, whereby composite electrical signals of low audio frequency and high audio frequency are produced that simulate the frequencies of the noises produced by an airplane in ight.

2. An electronic device as set forth in claim l wherein said band pass filter selectively passes the harmonics lying between 100 and 250 cycles and attenuates frequencies below 100 cycles and above 250 cycles, and said network having an adjustable frequency characteristic attenuates the high frequency components produced by said gas-filled triode filtered circuit, and attenuating means in each spectrum generating circuit for selectively controlling the propeller or air turbulence frequency characteristic and signal level of said simulated aircraft noise frequencies.

STANLEY SMITH STEVENS.

REFERENCES CITED The following references are of record in the file of this patent:

Number 6 UNITED STATES PATENTS Name Date Trouant Mar. 19, 1935 Roberts May 19, 1936 Engle Sept. 14, v1937 Dudley June 21, 1938 Ring et al July 11, 1939 Riesz Dec. 12, 1939 Dudley Mar. 19, 1940 Hilferty July 9, 1940 Guanella. Aug. 26, 1941 Purington July 11, 1944 Owens Aug. 1, 1944 Grieg Feb. 25, 1947 Labin Feb. 25, 1947 

